The present invention relates generally to water heaters. In particular, the present invention relates to gas-fired hot water storage heaters.
In a typical storage water heater (referred to herein simply as a water heater), water is heated in a tank where it is stored in a heated condition so that, when the demand occurs, the heated water is immediately available for use. As water is drawn from the tank, cold water enters, mixing with the remaining hot water in the tank. The mixture is then brought to the preselected temperature. Storage water heaters are useful in homes and many businesses, where the demand is high during certain times of the day and low or nonexistent during the balance of the day.
Another type of water heater is an instantaneous water heater, which typically holds ten gallons or less at any one time. Instantaneous water heaters are useful in continuous demand situations, as are present in many hotels and businesses.
Hot water storage heaters may be heated from the heat of combusted gas such as natural gas, propane, or butane or by electrical resistance heating. Natural gas is cheaper than electricity, but electrical resistance heating is more efficient since all of the heat produced enters the stored water. Therefore, a highly efficient gas-heated water storage heater is the most economical to operate.
There are significant differences between commercial and residential water heaters. In fact, there is a national industry standard, ANSI Z21.10.1, for residential water heaters and a different standard, ANSI Z21.10.3, for commercial water heaters. Commercial water heaters are classified as those that are rated at more than 75,000 Btu/hr; residential water heaters are classified as those that are rated at 75,000 Btu/hr or less. Also, residential water heaters heat water to a temperature no higher than 160.degree. F.; commercial water heaters heat to 180.degree. F.
There are other differences as well. The recovery rates, standby heat loss rates, and efficiencies of commercial water heaters are faster, lower and higher, respectively than those of residential units. "Recovery rate" is the number of gallons of water the water heater can bring to temperature per hour and is usually a function of inlet water temperature and temperature setting. "Standby loss" is a measure of how much heat is lost over a twenty-four hour period without the addition of heat; standby loss is expressed in percents and is typically 2-3%. The efficiency of a water heater is a measure of how much heat from the combusted fuel is transferred to the water. Thus, a residential unit and a commercial unit may appear to be the same size. However, internally the commercial unit will heat water to a higher temperature and more quickly, and be made to be considerably more robust and efficient. Somewhat ironically, capacity is not a factor that distinguishes commercial from residential hot water heaters, since the capacity of both is typically 100 gallons or less.
Designing a water heater requires consideration of more than thermal efficiency. The cost of manufacturing the water heater is also important. Incremental efficiency increases will not always justify large changes in cost. Also, ease of installation and servicing are two other important factors in water heater design. Therefore, water heater designers must consider a number of factors, all too often conflicting factors, in making design decisions.
There are inevitably, then, a number of designs for water heaters. Most water heaters, however, comprise an insulated tank sized to hold a quantity of water, a source of heat, a water inlet and outlet, and a heat exchanger immersed in the water in the tank. Several structural features are generally common to water heaters or at least many water heaters, although the specific compositions, geometries and interrelationships of components of similar but not identical water heaters oftentimes result in radically different performances. For example, the heat exchanger is sometimes a tube formed into a coil through which the hot combustion gases flow, giving up much of their heat to the water surrounding the coil.
In U.S. Pat. No. 4,492,185, Kendall et al. show such a coil in a residential water heater. Their water heater includes a heat exchanger comprising a central tube that runs vertically from the top of the heater approximately halfway down, and that is then formed into a coil that continues to the bottom of the tank.
Other examples of water heaters with coils exist in the art. For example, U.S. Pat. No. 4,203,392 discloses such a design, with the additional feature of a horizontal plate placed within the interior of the tank, which defines an upper or "super heated tank" and a lower "reserve tank." In addition, U.S. Pat. Nos. 2,581,316 and 2,787,318 both advance water heaters having a spiral heating coil running the length of the tank interior.
Nonetheless, because of the quantity of hot water used in today's society, there remains a need for a high efficiency, cost-efficient, gas-fired commercial water storage heater.
In a normal combustion burner for use with water heaters, air and gas are mixed in a preselected ratio and transported at a preselected flow rate to an ignition means where the mixture is burned to produce a flame. The heat generated by combustion is transferred, by convection, through a vertically-oriented heat exchange tube to the water in the tank.
The use of these heat exchange tubes in conjunction with existing combustion burners provides inefficient results. Most existing burners produce an unfocused, "bushy" flame shape. This unfocused flame shape often contacts the interior wall of the heat exchange tube, which in turn leads to the thermal degradation of the interior lining of the heat exchange tube. Moreover, contact of the flame with condensed water, residing on the interior of the heat exchange tube, results in inadequate combustion efficiency.
Another problem not properly addressed by existing burners is the inadequate mixing of the air and gas prior to combustion. Inadequate mixing of the fluid components results in an erratic flame shape, inefficient combustion, and often results in the inability to maintain a flame, commonly referred to as a "flame out."
Still another problem common to present day combustion burners is the inability of the burner to achieve various thermal ratings without changing the air blower or altering its power consumption. The thermal rating of a burner, measured in Btus/hr, is largely a function of the air flow rate. Consequently, when it is necessary to increase the thermal rating of the burner, the air flow rate must also be increased. This increased flow rate is accomplished by increasing the voltage consumption of the air blower, thereby enabling the transportation of air at a greater flow rate. If the voltage of the air blower cannot be increased, the air blower is usually replaced with one having greater air flow capacity. Neither of these solutions is satisfactory, since both increase cost, the former in terms of operating costs and the latter in replacement costs.
Therefore, there exists a need for a combustion burner for a water heater that provides sufficient mixing of air and fuel, produces a narrow, long focused flame, and that achieves various thermal ratings without altering the characteristics of the air blower.